1. Field of the Invention
The invention relates to metal-faced composite tooling, and more particularly to composite backup structures supporting a thermally-sprayed metal shell.
2. Historical Perspective
The design and fabrication of tooling used for the fabrication of advanced composite components forms one segment of the advanced composite structure industry. "Tooling" refers to the mold that forms the composite. Advanced composites are typically defined as thermosetting or thermoplastic matrix materials containing a high content of high strength, continuous, reinforcing fibers such as carbon, graphite, kevlar, or fiberglass. Advanced composites typically are used in high performance structural applications because of their higher specific strength and stiffness as compared to earlier composite materials. Advanced composite structures are generally more expensive than structures constructed of traditional metals such as aluminum or steel. A majority of this cost disadvantage is due to the cost of tooling and processing. The present invention overcomes this disadvantage by providing tooling for fabrication of composite products that is of lower cost, lighter in weight, and available in less time, thereby allowing the industry to provide products at a lower cost. It also reduces the expense and long lead times of product development programs.
There are two basic processes for constructing tooling for composites manufacturing. One method is hard metal tooling. This method consists of directly fabricating or machining the final tool from the tool drawing. The second method relies on the construction of intermediate models or patterns of the part for which the tooling is being constructed. Typically a plastic, wood or plaster model of the final part is constructed from part drawings. Such original model is referred to as the master pattern. Metal-faced composite tooling, also known as spray metal tooling, is made from a thin metal shell formed by spraying the metal on the master pattern, which metal shell or master pattern is then suitably reinforced and removed to be used a mold.
The present invention presents a new design of high performance composite as a backing structure for such thin metal shell that is capable of maintaining mold stability during fabrication and the mold's subsequent use when it is subjected to high temperatures and compaction pressures as are experienced during the molding of advanced composite structural components.
3. Prior Art
Spray metal tooling technology was introduced some forty years ago. Recently, however, with an increasing interest and utilization of advanced composite structural components, it has become increasingly important to provide tooling that will be stable, that will not distort or significantly deform, over a number of applications of high heat and pressure. Specifically, prior to the present invention, it has not been possible to provide, in a mold backing, for mold stability at temperatures of up to 750.degree. F. and compaction pressures as high as 1500 psi, that are or may be required for processing advanced composites.
A U.S. Pat. No. 3,533,271, to Walkey, illustrates an early approach to a die fabricated by spray metal techniques. This patent shows filling a die shell with a binder that solidifies as a tool backing. It also teaches forming a backing material from combinations of: an epoxy resin and shot; a mix of iron shot and sodium silicate, melted to form a glass like material; epoxy impregnated glass fibers; and aluminum and plastic. All of which materials, except for the sodium silicate and shot composite, have been limited to use with tooling that operates at temperatures at or below 250.degree. F. A sodium silicate and shot mix has been used at temperatures of approximately 1450.degree. F., that exhibited greatly different coefficients of thermal expansion of the two materials. Accordingly, with only a few cycles of heating and cooling, the materials will have separated, thereby compromising the backing material's structural integrity. At temperatures of 350.degree. to 750.degree. F. and pressures of up to 1500 psi, as are or may be required for molding or laminating advanced or high performance composites, such as epoxy, bismaleimide, polyamide and thermoplastics, earlier backing or reinforcing materials have quickly failed, due to thermal stresses as occur in repeated thermal cycling.
The present invention provides, through a utilization of treated small, short and long fibers in an appropriate resin mix a backing structure that is strong and stable. Which backing structure will exhibit a low coefficient of thermal expansion at temperatures of from 70.degree. F. to 750.degree. F. and compaction pressures of up to 1500 psi and good characteristics of thermal diffusivity and low density. Additionally, the backing material of the present invention will exhibit minimal linear shrinkage of only 0.025% to 0.05% in both formation and with heating and cooling, as compared to approximately 1-1.5% for resin alone.
It is, of course, known that short fibers, sometimes referred to as whiskers, microfibers, mineral fibers, chopped fibers, milled fibers, short metal and metal coated fibers, that are from submicron to 7 to 13 microns in diameter, will exhibit high mechanical properties to a mold and will increase the stiffness of a resin. Such mix is known to produce a composite having a greater strength than does a neat resin composite. However, such composites have high density, poor thermal conductivity, and exhibit unacceptable shrinkage during fabrication. Papers given by John V. Milewski entitled "How to Use Short Fiber Reinforcements Efficiently" and "Problems and Solutions in Using Short Fiber Reinforcements" to the 37th Annual Conference, Reinforced Plastics/Composites Industry, The Society of the Plastics Industry, Inc., Jan. 11-15, 1982, summarize the characteristics, uses and limitations of short fibers in resin composites.
Distinct from the earlier composites, the present invention, utilizes a mix of small, short and long fibers, that are treated for improved resin wetting, thereby reducing initial shrinkage, and producing a tool backing having a high flexural strength with a small coefficient of thermal expansion, good thermal diffusivity, and low density.